In internal engines, technical improvements for higher power, higher durability, higher fuel efficiency, etc. are required. Technical improvement for higher power results in higher temperature and pressure in engines. Particularly, top rings used in a high-temperature, high-pressure environment are strongly required to have high wear resistance on peripheral sliding surfaces and side surfaces.
The wear resistance of piston ring side surfaces has a close relation to microwelding occurring when combined with an aluminum piston. Patent Reference 1 discloses a piston ring having a phosphate or triiron tetraoxide coating formed as a primer coating on at least a lower surface, and a solid-lubricant-containing, heat-resistant, wear-resistant resin coating formed on the primer coating to prevent microwelding to the side surfaces. It is described that the above primer coating is formed to increase the adhesion of the resin coating. Patent Reference 2 points out that the primer coating of phosphate or triiron tetraoxide disclosed in Patent Reference 1 deteriorates the fatigue strength of the piston ring, disclosing a piston ring having a nitride layer in place of the phosphate or triiron tetraoxide coating, and a solid-lubricant-containing, heat-resistant, wear-resistant resin coating formed on the nitride layer.
With respect to a heat-resistant, wear-resistant resin coating containing a solid lubricant, Patent References 1 and 2 disclose the use of tetrafluoroethylene resins, oxybenzoyl polyester resins and polyamideimide resins, and Patent Reference 3 discloses the use of polyamideimide-silicon dioxide hybrid materials and polyimide-silicon dioxide hybrid materials.
Patent Reference 4 discloses a hard carbon coating containing 0.5 atomic % or more and less than 5.0 atomic % of one or more elements selected from the group consisting of Si, Cr and Ti for exhibiting excellent resistance to microwelding of a piston ring to a piston, in place of the lubricant-containing, heat-resistant, wear-resistant resin coating.
However, the above resin coating has absolutely low wear resistance, failing to exhibit microwelding resistance for a long period of time, though it is effective to prevent initial microwelding. Also, the hard carbon coating cannot always be put into practical use in any situation in view of the cost, despite longer life than the resin coating.
In view of the above technical background, a nitride layer is generally formed on piston ring side surfaces to secure wear resistance. For this purpose, martensitic stainless steels containing large amounts of nitride-forming elements for easy formation of the nitride layer, such as SUS440B (17% Cr) and SUS420J2 (13% Cr), are conveniently used, but low-alloy steels such as silicon-chromium steels are not usable because of insufficient nitridability.